This invention relates to a new process to recover the precious metals, such as gold, silver, selenium and tellurium, etc. from copper refinery anode slimes.
Copper refinery anode slimes, briefly called anode slimes, which are settled and accumulated at the bottom of electrolytic copper refining cells, are made up of those components of the anodes which are not soluble in the electrolyte. They containing varying quantities of copper, silver, gold, sulphur, selenium, tellurium, lead, arsenic, antimony, nickel, silica, iron, etc. The object of treating anode slimes has always been the recovery and separation of precious metals. With the passage of time, increasing attention has been paid to improving the recovery and quantity of by products, mainly selenium and tellurium.
The conventional treatment of anode slimes entails numerous, and often complex, hydro- and pyrometallurgical operations. These result in the removal of the various constituents over a number of process steps, the nature of the particular anode slimes being treated determining the final procedure adopted.
In general existing anode slimes treatment processes have been designed to remove copper first, and then to recover the precious metals in the form of an alloy, referred to as Dore metal. Impurities such as selenium constitute useful by-products. Whilst recoveries of silver, gold and copper are high at about 97 percent, the recovery of selenium seldom exceeds 80 percent. Also, because of the tendency for selenium to oxidise and volatilise at fairly low temperatures, it is evolved during all heat-treatment stages and its collection is thus spread over the entire process.
Copper present in the slimes occurs usually as metallic copper dislodged from the anodes, together with copper sulphides, selenides and tellurides. Depending on how effectively the slimes have been washed after being removed from the electrolytic cell, they may contain up to 25 percent of the copper as water-soluble sulphates. The usual practice is to remove the copper as the first stage in the slimes treatment process. This is accomplished by carrying out a low temperature (260.degree. to 430.degree. C.) oxidizing roast followed by sulphuric acid leaching. Alternatively, the slimes may be digested at about 260.degree. C. for 8 to 12 hours with concentrated sulphuric acid to decompose the selenides and the reaction completed by roasting the mass. The soluble copper sulphate is then separated from the calcine by leaching with water. The copper sulphate solutions obtained by these methods are usually contaminated with selenium and silver, which must be removed before the copper is finally recovered by electrowinning, cementation or crystallisation as copper sulphate.
Selenium occurs in the slimes both as metallic selenides and as the element. It is generally recovered by smelting the decopperized slimes with soda or nitre and dissolving the resulting water-soluble sodium selenite from the crushed slag. Elemental selenium is then precipitated by passing sulphur dioxide gas through the acidified leach solution.
Other important methods include roasting with concentrated sulphuric acid, when the selenides are converted to selenium dioxide which volatilises and is collected in wet scrubbers and elecrostatic precipitators. Selenium is precipitated from the scrubber solutions as described above, purified by dissolution and re-precipitation and finally distilled to obtain a readily marketable product.
The decopperized anode slimes, containing varying amounts of selenium, are smelted in a Dore furnace, the major impurities such as lead, arsenic, selenium and copper being slagged off with silica, soda and nitre. The resulting Dore metal, containing the gold, silver and platinoids, is then electrolytically refined in a Moebius. Silver of "three nines" quality or better is recovered at the cathodes, while the gold and platinoids, together with some silver, settle as an anode sludge. The sludge may be treated by boiling with sulphuric acid to dissolve the silver and high purity gold may then be recovered from the residue by smelting and electrorefining.
The above mentioned conventional pyrometallurgical process to recover the precious metals is getting harder and harder to operate due to the facts that energy costs have soared, the strict pollution laws have been enacted and high grade ores have been depleted.
After an intensive investigation into the problems and limitations of conventional pyrometallurgical process for the treatment of copper refinery anode slimes, we have now designed a new process for these anode slimes that provides a clean separation of the valuable constituents of the slimes without producing a large amount of off-gas as well as wasting a large amount of energy.
In order to cope with the soaring energy costs, to comply with strict pollution laws and to face up to the depletion of high grade ore, a new hydrometallurgical process to recover the precious metals has been developed and tested successfully. The process is economic, energy-saving as well as pollution-free. The newly developed hydrometallurgical process consists of (1) leaching step, where the precious metals are introduced to the aqueous environment, (2) solvent extraction step, where the metals are separated and purified, and (3) reduction step, where the metallic ions are reduced and precipitated from solutions.